With SPF 30 sunscreen, I am usually protected for an entire day and wont be getting more than a little darker. Now, I am getting low on SPF 30 cream but I have a large supply of SPF 15. Can I just use that one twice often to make up for the difference?

How does SPF 15 and SPF 30 sunscreen relate? Is it additive in strength or duration? Meaning can 15 be applied twice as often or twice as much? Or is the protection different?

Note that I am looking for how the various protection levels relate.

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    It has to do with the amount of sun blocked not the length of time it lasts. I don't have a chart handy but the "sun protection factor" isn't something simple like SPF 30 is twice as strong as SPF 15. I recall reading something a while back that said roughly SPF 50 is as high as is practically needed because it blocks something like 99% of damaging rays. – Erik Mar 14 '17 at 0:18
  • @Erik - That is in fact my question. What is the relationship? – Itai Mar 14 '17 at 1:30
  • I wrote an answer on the question that @CharlieBrumbaugh linked to since your question is a duplicate. – Erik Mar 14 '17 at 2:48

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